This invention relates to a single-mode optical fiber, and more particularly to an improvement of an optical fiber having a single-polarization characteristic.
Various types of optical fibers and methods of manufacturing the same have been proposed, that can propagate light polarized in a definite direction by providing a difference between propagation constants .beta.x and .beta.y of the modes propagating in orthogonal directions, that is by imparting a polarization preserving characteristic to the fiber. However, until today an optical fiber having an excellent polarization preserving characteristic, low loss and a long length is not yet available.
For example, in order to obtain a single-polarization, single-mode optical fiber, the core is shaped to have an elliptical cross-sectional configuration to afford the polarization preserving characteristic. An optical fiber having such a construction is prepared by grinding opposing surfaces of a rod shaped preform comprising a core and a cladding with a modified chemical vapor deposition (MCVD) method to form parallel ground surfaces, then applying a jacket onto the ground preform for adjusting the core diameter and then draw or elongate the jacketed preform by heating the preform at a temperature above 2000 degrees in centigrade in a heating furnace. More particularly, for drawing, the jacketed preform is heated so that the viscosity of the assembly is lowered and the drawn fiber would have a circular surface owing to surface tension. Consequently, due to the change of the shape of the flat portions, the completed fiber will have an elliptical cross section. Since the elliptical cladding has different wall thickness around its periphery and since the thermal expansion coefficient of the cladding is larger than that of the jacket, stress is applied to the core thereby producing an optical fiber having a polarization preserving characteristic.
An optical fiber having such construction is disclosed in a V. Ramaswamy et al paper of the title "Single Polarization Optical Fibers: Exposed cladding technique", Applied Physics Letter Vol. 33, No. 9, Nov. 1, 1978, pages 814-816.
However, in an ordinary optical fiber, the light propagating through the core more or less diffuses into the cladding (for example, about 15-25%) so that the fiber is liable to be influenced by the contained in the cladding. With the construction described above, however, since the thickness of the cladding is not uniform, it is difficult to obtain an optical fiber having a polarization preserving characteristic and a low loss characteristic.
Since a portion of the elliptical cladding having a large thermal expansion coefficient and extending in the minor axis direction partially cancels the stress induced by the cladding and extending in the direction of the major axis of the ellipse, the polarization preserving characteristic is degraded.
Furthermore, as a process that mechanically grinds the side surfaces of the preform in the longitudinal direction is used, the working accuracy is not uniform. This makes it difficult to obtain a long optical fiber. Such grinding fractures the preform during the grinding step, thus decreasing the yield of satisfactory product.